Question Mark Folder appears when iMac turned on

I purchased a new optical drive from a reputable mac seller in my area and installed it due to the old one not reading disks anymore. I was very careful and followed the instructions according to an article I found on xlr8yourmac.comarticle I found on xlr8your mac

However, now when I start the computer up, I get the question mark folder. I've done a little research but thought I' check the Macrumors forum before I spent all day on this. The store where I bought the new optical drive from doesn't open for another few hours so, here I am.

so, I replaced the old optical drive and the computer booted up just fine. I swapped the NEW drive back in and got the same old question mark icon. I'm guessing the computer needs to recognize the new optical drive, but I don't know how to do this.

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